Helical scan recording apparatus include a scanner drum assembly, wherein a magnetic head is mounted within an upper drum which rotates at high speed relative to a longitudinally moving tape which is helically wrapped about the drum assembly, so as to scan tracks that are arranged in parallel segments diagonally across the tape length. To cause the magnetic head to follow a previously recorded track, it has been usual to mount the head on the end of a supporting blade that pivots or flexes transversely to the direction of head rotation and thus tape movement, in response to a magnetic drive. A tracking signal is derived from the continuously rotating flexible head to determine its lateral position with respect to the track being read. The tracking signal provides an indication that the head is either exactly centered on the track, or is off center on either side of the track. This tracking signal then is used in a closed position servo loop, in conjunction with a separately measured head displacement signal of a closed displacement servo loop, to control the pivoting or flexing of the head supporting blade to keep the head centered on the track.
Because the strength of the tracking signal is directly related to a precise alignment of the flexible head with the center of the track, it is a critical operating parameter of the scanner assembly that the position of the head be precisely known relative to a nominal centered displacement, as well as to the track center. Thus, it is important that the head displacement measurement is accurate and free of noise and phase shift over a wide bandwidth, so that it may be used in the closed position servo loop which is driven by the tracking error signal. The combined position and displacement servo loops correct any tracking errors.
The measured displacement is used for two purposes. The first is to provide a feedback signal which allows the closed position servo to be constructed for the purpose of providing uniform response over a wide range of frequencies. The second is to provide a linear and known relationship between head displacement and voltage. This allows the head to be offset by a known amount for the purpose of "jumping over" tracks as needed in slow and fast motion playback.
However, typical head displacement measurement techniques determine lateral head displacement by employing a light source in conjunction with a moving slit secured to the flexible blade which supports the head. The movement of the slit varies accordingly the amount of light from the light source which impinges either of a pair of photo detectors. The amount of light which is detected by one or the other photo detector is proportional to the degree of lateral movement of the blade supporting the magnetic head. Thus the comparative values of the signals generated by the two photo detectors is indicative of the displacement of the head itself from a nominal centered position.
However, typical measurement techniques experience several inherent disadvantages. For example, a typical arrangement places the light source and the photo detectors within a support housing which secures the flexible head assembly in the rotatable upper drum, thereby substantially complicating the circuitry extending into the region of the head assembly. In addition, placing the light source and the photo detectors in the proximity of the head supporting member is undesirable since space is at a premium in the support housing which contains the flexible head assembly. Further, because of the optical arrangement of the light source, the slit, and the detectors relative to the head assembly itself, the slit and the photo detectors "see" a different portion of the light generated by the light source. That is, as the head is deflected over its range of transverse displacement, a varied gradient of the light generated by the surface of the bulb is used as the working part of the beam, causing non-linearities in the resulting head displacement signal. Moreover, in such a typical arrangement, an electromagnetic voice coil included in the driving mechanism of the automatic scan tracking system is located closely to the photo detectors. This, in turn, results in undesirable crosstalk between the voice coil of the automatic scan tracking system and the photo detectors of the head displacement measurement apparatus.
Accordingly, it would be highly desirable to remove the light source and the photo detectors from within the support housing of the flexible head assembly. Further it would be desirable to move the light source and the photo detectors away from the voice coil of the automatic scan tracking mechanism so as to substantially reduce the cross talk between the head displacement measurement apparatus and the automatic scan tracking system. Also, it would be desirable to modify the light source to provide a small or zero gradient of light across the full displacement range of the automatic scan tracking head, thereby eliminating non-linearities in the head displacement signal.